The present invention relates to heat sinks, and more particularly to a heat sink for a magnetically tuned very high frequency filter to conduct heat from the interior of a magnetic housing at the point of greatest heat concentration to a point external of the magnetic housing.
Magnetically tuned filters generally have one or more resonant crystal spheres located in respective cavities of a filter housing. The filter housing is enclosed within a magnetic housing with an appropriate maagnetic coil to generate a magnetic field which is used to tune the crystal spheres to a desired frequency. Current magnetically tuned filters use crystal spheres of yttrium-iron-garnet (YIG) which provide a frequency range from about 2 GHz to approximately 21 GHz. Up to these frequencies the heat generated by the current in the magnetic coils does not rise to a level sufficient to require any extraordinary heat dissipation techniques. However, since the power required is a function of the square of the frequency, at frequencies above 21 GHz the heat generated rapidly becomes significant. In fact as the heat rises significantly the resistance of the coils increases, adding to the heat generation so that the heat generated becomes approximately a function of the cube of the frequency. For example at a 40 GHz frequency the temperatures can reach 200.degree. C. depending upon the gap between the pole pieces.
The frequency of the resonant crystal spheres is sensitive to the field strength of the generated magnetic field. The field strength depends on the current in the magnetic coils and the gap between the pole pieces. In many applications, such as in spectrum analyzers, the magnetically tuned filter is subjected to cyclical operation, i.e., the frequency is swept across the range continuously as often as one thousand times per second. This provides a concomitant thermal cycle in the pole pieces which causes expansion and contraction so that the gap between the pole pieces and the crystal spheres varies. The resulting frequency output, therefore, is unpredictable unless the heat can be dissipated so that the gap remains relatively constant.
Attempts to dissipate this heat have been made by placing the magnetic housing upon an external heat sink so that the heat is conducted through the magnetic housing to the heat sink. This is unsatisfactory since the magnetic housing is generally a poor thermal conductor and the maximum heat is concentrated by the coils at the center of the magnetic housing adjacent to the pole pieces. Another approach is to try to maintain a thermal equilibrium between the pole pieces and the outer portion of the magnetic cup by short circuiting the pole pieces to the magnetic housing with an aluminum plate. This approach does not dissipate the heat, but does provide thermal equilibrium so that the tolerance changes in the gap can be determined and compensated to a certain extent.
What is desired is a heat sink for a magnetically tuned filter operation above 21 GHz which dissipates the heat generated by the magnetic coils while maintaining thermal equilibrium within the magnetic housing.